The present invention relates generally to a paper substrate for an inkjet recording sheet, and more particularly to a basecoated paper substrate designed to accommodate an ink receptive coating.
Conventional inkjet recording sheets are prepared by coating a paper surface with a coating composition containing a porous fine pigment in a binder capable of accepting and retaining the ink on the surface of the paper, while the paper absorbs the ink vehicle. However, in some instances the paper is unable to instantaneously absorb the entire amount of the ink vehicle. This drawback often results in spreading of the ink drops, which creates resolution and sharpness defects, and unduly long drying times.
It is known that inkjet recording paper may be improved by first applying a suitable base coat having good absorption characteristics to the paper, before applying the ink receptive top coat. Such base coatings must possess a balance between good coating holdout and adequate water absorbency. It has also been discovered that such basecoats must be compatible with the subsequently applied ink receptive top coatings for good runnability and performance during manufacture.
Substrates for inkjet recording sheets must rapidly absorb the vehicle of the inks to reduce drying time with little or no backside show-through. Further, the substrate plays a role in preventing diffusion of ink laterally on the surface of the recording sheet in order to achieve high resolution without blurring. Thus, for obtaining color images having good color density and resolution, with good absorptivity and water fastness as well as optical brightness, the substrate plays an important role. Paper substrates for ink receptive coatings are generally made from bleached chemical pulp to which fillers, dyes, and if required, sizing agents and strength enhancers are added. An example of a typical paper substrate for use in the manufacture of an inkjet recording sheet is disclosed in pending U. S. patent application Ser. No. 09/020,826 filed Feb. 9, 1998, now U.S. Pat. No. 5,985,424 owned by the present assignee herein.
It is also known that paper substrates for inkjet recording can be improved by applying a basecoat to the paper before applying the ink receptive top coating. Such base coats generally comprise a pigment and binder to provide a surface having a porous structure which has good absorptivity for the ink vehicle. Examples of base coatings for inkjet recording sheets are disclosed, for example, in prior U.S. Pat. Nos. 4,474,847; 5,171,626; 5,670,242; 5,747,148; and 5,759,673. In particular, the xe2x80x2242 patent discloses typical examples of the materials useful for the base coatings of inkjet recording sheets to include various known pigments used in ordinary coated papers, such as kaolin clay, calcined clay, amorphous silica, zinc oxide, aluminum oxide, aluminum hydroxide, calcium carbonate, satin white, aluminum silicate, smectite, magnesium silicate, magnesium carbonate, magnesium oxide and others. However, in practice, the most often used pigments for such basecoats comprise clay, calcium carbonate, amorphous silica and aluminum oxide. Another requirement of the basecoat is that the coating components not interfere in any way with the top coatings which provide the ink receptive surface for such products. Where adverse reactions occur, the base coat components must be changed or altered.
In the aforementioned pending U.S. patent application Ser. No. 09/020,826, a base coating is disclosed which comprises as the coating pigment precipitated calcium carbonate, calcined clay, and, if desired, titanium dioxide. Polyvinyl acetate is disclosed as the preferred binder. This base coating has been found to be compatible with most top coatings, and when combined with a top coating containing fumed silica, achieved satisfactory printing results with regard to ink bleed, drying time and gamut characteristics. However, the same base coating produced undesirable interactions when used with other top coatings. It was observed that the precipitated calcium carbonate in the above described base coating had a tendency to destabilize top coatings which were acidic in nature. Thus to overcome this interaction, and to provide a more universally acceptable base coating, the pigment combination disclosed herein was adopted. By substituting a kaolin clay based pigment in place of the precipitated calcium carbonate pigment utilized in the above described base coating and increasing the calcined clay content, the destabilization mentioned above with the use of acidic top coatings was averted while still achieving the desired inkjet printing characteristics.
Thus while it was heretofore known that the selection of a suitable base coating is necessary to obtain proper inter color bleed, gamut and ink drying time of the final sheet, it has now been found by the present invention that the proper selection of the base coating components is also necessary to prevent any undesirable interactions between the base coating and the top coating during the manufacturing process.
It is, therefore, a general object of the present invention to provide a basecoated substrate for an inkjet recording sheet that has superior performance during inkjet printing.
It is a further object of the invention to provide a coated paper basestock for an inkjet recording sheet which has excellent dimensional stability when used with aqueous based inks.
A further object of the present invention is to provide a base coating for a substrate useful in the manufacture of inkjet printing paper that has little or no adverse interaction with the top coatings applied thereto.